https://en.wikipedia.org/wiki/Bag_valve_mask
I have to assume this is more reliable than whatever somebody hacked together in their garage. Your average builder may be able to make something that works OK in testing, but having it work 24/7 by people unfamiliar with its quirks is a different matter entirely.
The official word out of New York is that they are going to use bag valve masks when they run out of ventilators and train people how to squeeze the bag.